Jonathan AC Roberts (6/21/2010)
Both methods achieve the same thing but using certificates is generally recommended. Some things are different for example if there is auditing a table with a trigger and the function user_name() is used then the function would return the EXECUTE AS user not the actual caller of the stored procedure. Also, if another stored procedure is called from within the stored procedure it would execute that stored procedure with the EXECUTE AS user rather than the actual user.
You can, and probably should use the ORIGINAL_LOGIN() function in any logging you do in triggers so that you can get the actual user in the cases of EXECUTE AS.